LM308A
SUGGESTED DESIGN APPLICATIONS
INPUT GUARDING
Special care must be taken in the assembly of printed
circuit boards to take full advantage of the low input currents
of the LM308A amplifier. Boards must be thoroughly cleaned
with alcohol and blown dry with compressed air. After
cleaning, the boards should be coated with epoxy or silicone
rubber to prevent contamination.
Even with properly cleaned and coated boards, leakage
currents may cause trouble at +125°C, particularly since the
input pins are adjacent to pins that are at supply potentials.
This leakage can be significantly reduced by using guarding
to lower the voltage difference between the inputs and
adjacent metal runs. The guard, which is a conductive ring
surrounding the inputs, is connected to a low–impedance
point that is at approximately the same voltage as the inputs.
Leakage currents from high voltage pins are then absorbed
by the guard.
Figure 8. Sample and Hold
VCC
1.0 M
Input
150 pF
1
6
LM101A (3)
or equiv
Q1
Sample
Output
Q2
1.0
µF
(1)
30 pF
Output
Figure 7. Fast (1) Summing Amplifier with
Low Input Current
C5 (2)
R1
R4
0.002
µF
LM308A
RS
Input
150 k
0.002
µF
R2
1M
2
3
1.0 M
Compen B
300 pF
(1) Power Bandwidth: 250 kHz
Small Signal Bandwidth:
3.5 MHz
Slew Rate: 10 V/µs
(2) C5 =
6 X 10–8
R1
(3) In addition to increasing speed,
the LM101A raises high and low
frequency gain, increases output
drive capability and eliminates
thermal feedback.
(1) Teflon, Polyethylene or Polycarbonate
Dielectric Capacitor
Figure 9. Connection of Input Guards
Inverting Amplifier
R1
Input
R2
R3 (1)
R3 (1)
Output
Output
R3 (1)
C1
C1
(1) Used to compensate for large source resistances.
C1
Input
R1 R2
Note:
must be an impedance.
R1 +R2
Input
R1
Output
Follower
Noninverting Amplifier
R2
4
MOTOROLA ANALOG IC DEVICE DATA
ONSEMI [ ON SEMICONDUCTOR ]
